At the launch of the report João Breda, head of the WHO European office for the prevention and control of noncommunicable diseases, said that public health experts were using "obsolete tools" to fight the rising tide of advertisements.

"We are using the wrong ammunition for a very significant problem. These technological innovations [used by advertisers] make our restrictions void - they no longer work in this context. The digital world is wonderful for marketeers - it's more targeted and they can get much more data," he said.

But as the amount of time children spend online increases, there has been growing concern that they are faced with a deluge of advertisements and marketing messages pushing food and drink that is high in fat, sugar and salt.

The WHO report highlights the lack of research into children’s online lives as well as the dearth of data quantifying the amount of marketing of unhealthy food, alcohol and tobacco to children and where and when they are seeing it.

"We need to know more about children to understand how to protect them in the best possible way. We need to know how old they are, whether they are boys or girls, where they are from, their social economic status. We even need to know what sort of digital device they use,” said Dr Breda.

The report warns that parents struggle to be "effective gatekeepers” of their children’s online lives, with many assuming that their children are savvy enough to ignore marketing messages. The shift to using social media on mobile screens means that parents are now even less likely to know when their children are being exposed to certain types of ads.

Mimi Tatlow-Golden, lecturer in developmental psychology and childhood at the Open University, told the launch that it was not a "fair fight" between regulators and social media companies.

"This is a David and Goliath situation," she said. "We know almost nothing and the people who are running the systems know everything. The consent concept is a fiction - we're talking about wholesale, non-consensual data extraction from children," she said.

In the UK the Advertising Standards Authority introduced new regulations banning advertising of junk food when children make up 25 per cent or more of the audience. The rules, which came into force last summer, encompass all types of media.

But advertisers can get around these regulations, said Dr Tatlow-Golden.

"It's easy for them to say we're not targeting children - we're just sending messages to people who like Ariana Grande [a singer with a huge following among children and teenagers]," she said.

Last week food giant Mondelez was forced to remove a downloadable comic and audiobook called The Missing Hop, which featured the tale of Freddo, a chocolate frog.

The WHO report highlights the ASA regulations as an example of good practice although obesity campaigners in the UK have said that they do not go far enough.

In response to the ASA's Freddo ruling Barbara Crowther, co-ordinator of the Children's Food Campaign, said there were "no major disincentives, no meaningful sanctions and no punishments” for companies that break the rules.